1. Field of the Invention
The present invention relates to an illumination optical system which has a light source including a plurality of light emitting sections and an image projection apparatus which is provided with the illumination optical system.
2. Description of the Related Art
An image projection apparatus such a projector modulates illumination light from a light source using a spatial light modulation element such as a liquid crystal panel or a micromirror array device to display an image by projecting the modulated light onto a plane to be projected such as a screen.
As a light source used for the image projection apparatus, generally, a high-intensity discharge lamp such as an extra high pressure mercury lamp, a metal halide lamp, or a xenon lamp is used. However, in the case of using the high-intensity discharge lamp, the apparatus grows in size because a cooling system is necessary for releasing heat, or the exchange of the lamp at a constant lighting time is necessary because the life of the lamp is short. Therefore, as a light source, the use of a semiconductor light emitting element, which does not need a large cooling system and has a life longer than that of the lamp, has been required.
However, in the case where the semiconductor light emitting element is used as a light source at present, a plurality of semiconductor light emitting elements need to be arrayed for obtaining adequate light intensity.
On the other hand, in order to illuminate a spatial light modulation element, i.e. a plane to be illuminated, with uniform illumination by light from a light source, an illumination optical system using a fly-eye lens including a plurality of lens cells, a rod lens, or the like has been proposed.
For example, Japanese Patent Laid-Open No. 3-111806 discloses that light from a light source is divided into a plurality of light beams by a first fly-eye lens and the plurality of light beams are overlapped on a spatial light modulation element by a second fly-eye lens and a condenser lens to uniformly illuminate a plane to be illuminated.
An array light source having a plurality of light emitting sections can also be used as such an illumination system. For example, Japanese Patent No. 3987350 discloses an illumination optical system which has a light emitting section provided for each lens cell of a fly-eye lens in an array light source, and uniformly illuminates a plane to be illuminated by changing a light beam entering position from each light emitting section to each lens cell.
Japanese Patent Laid-Open No. 2002-184206 discloses an illumination optical system which uniformly illuminates a plane to be illuminated on condition that a divergent light beam from each light emitting section is paralleled by a lens array in an array direction of light emitting sections of an array light source and that the light beams from a plurality of light emitting sections enter each lens cell of a fly-eye lens.
Furthermore, Japanese Patent Laid-Open No. 2004-220016 discloses an illumination optical system which uniformly illuminates a plane to be illuminated by paralleling a divergent light beam from each light emitting section of an array light source by a parallel lens and by dividing the parallel light beam by a plurality of lens cells.
Like illumination optical systems disclosed in Japanese Patent No. 3987350, Japanese Patent Laid-Open No. 2002-184206, and Japanese Patent Laid-Open No. 2004-220016, a light intensity as a whole array light source can be increased by using the array light source even if a light intensity of each light emitting section in the array light source is small. Furthermore, a plane to be illuminated can be uniformly illuminated if the light emitting intensity distribution of each light emitting section of the array light source is uniform and the light emitting intensity is stable.
However, the light emitting intensity distribution of an LED or a laser diode (LD) that is a semiconductor light emitting element is a Gaussian distribution. Furthermore, each semiconductor light emitting element has a variability of the light emitting intensity due to the influence of a production error, a structural defect, temperature fluctuation, or the like. Therefore, even in the illumination optical systems disclosed in Japanese Patent No. 3987350, Japanese Patent Laid-Open No. 2002-184206, and Japanese Patent Laid-Open No. 2004-220016, it is actually difficult to uniformly illuminate the plane to be illuminated.
For example, as disclosed in Japanese Patent No. 3987350 and Japanese Patent Laid-Open No. 2002-184206, when a light beam from each light emitting section enters one lens cell without being divided, the Gaussian distribution can not be corrected. Therefore, although a uniform illumination is performed by overlapping Gaussian distribution light beams from a plurality of light emitting sections with a shift from each other on a plane to be illuminated, a high accurate adjustment is necessary for uniformizing the illumination at a joint section of light beams from a plurality of light emitting sections.
When there is a variability of light emitting intensity among a plurality of light emitting sections, local illuminance unevenness is generated on a plane to be illuminated because each of a plurality of light beams from the plurality of light emitting sections illuminates a different region on the plane to be illuminated.
It is also important for an illumination optical system installed in an image projection apparatus to be miniaturized. Like an illumination optical system disclosed in Japanese Patent Laid-Open No. 2004-220016, the influence of the variability of the light emitting intensity distribution of each light emitting section or the light emitting intensity among a plurality of light emitting sections can be reduced by dividing a plurality of light beams from a plurality of light emitting sections of the array light source by a plurality of lens cells. However, in an illumination optical system disclosed in Japanese Patent Laid-Open No. 2004-220016, the illumination optical system grows in size in an array direction of the light emitting sections because a divergent light from each light emitting section is paralleled by a parallel element corresponding to each light emitting section of the array light source to enter a fly-eye lens.